A high pressure discharge lamp lighting device for use in a light source device for back light, for example, of a liquid crystal projector is adapted to light up the lamp by converting a current supplied from a DC power source to a rectangular current at a predetermined lighting frequency thereby switching the lighting polarity of the high pressure discharge lamp.
FIG. 13 shows an existent lighting device 31 for a high pressure discharge lamp in which a DC power outputted from a DC power source 2 is inputted to a chopper circuit 3 and a control circuit IC converted into an appropriate DC current by controlling the duty ratio of a switching device 4 by a PWM control circuit 32 is inputted to a full-bridge circuit 5.
In the full-bridge circuit 5, paired transistors TR1, TR1, and Tr2, TR2 are alternately conducted by a full-bridge control circuit 33 at a frequency, for example, of about 100 Hz to form a lamp current IL comprising a low frequency rectangular wave form relative to a high pressure discharge lamp L started by an ignitor circuit 6 thereby lighting up the high pressure discharge lamp L.
By the way, a super-high pressure mercury lamp has been used in recent years instead of a high pressure mercury lamp as a light source such as for a liquid crystal projector.
Since the super-high pressure mercury lamp has an extremely high vapor pressure during lighting (about 106 Pa or higher), arc discharge is concentrated to the center of the discharge tube and both brightness and temperature are high, it has a merit of forming continuous spectrum, having color of light approximate to white, favorable color rendition and high emission efficiency.
However, in the existent high pressure discharge lamp lighting device of lighting only by the low frequency rectangular wave, since arc means can not be controlled in a super-high pressure mercury lamp, the amount of light entering the light receiving portion changes greatly by the arc movement and, when used to a liquid crystal projector, it causes a problem that the change of the screen illuminance increases to result in flickering on the screen.
Particularly, along with reduction in the size and the weight of liquid crystal projectors, reduction in the size and increase in the illuminance have been progressed for the reflection mirror of the light source and the size of the liquid crystal device as the photoreceiving portion is also reduced correspondingly, so that this results in a significant effect of flickering on the even for an identical amount of arc movement.
Flickering in a high pressure discharge lamp is formed due to the movement of the arc spot as a trigger point from which electrons are emitted when an arc directing from a cathode to an anode is generated.
Generally, in a case where the electrode temperature is uniform, a position for the nearest inter-electrode distance (protrusion formed at the top end of the electrode, etc.) constitutes an arc spot and, in a case where the distance is constant as in a parallel plate electrode, a position for the highest temperature constitutes the arc spot.
Accordingly, in a case of intending to control the arc spot, it may be considered that flickering can be suppressed by forming a high temperature portion locally on the electrode to fix the arc spot in a state where there is no temperature difference between the electrodes.
As a countermeasure, the present applicant has proposed a lighting device for a high pressure discharge lamp by supplying an AC lamp current upon lighting up a high pressure discharge lamp, of supplying a rectangular short period current at a frequency higher than that of a rectangular standard period current at a predetermined lighting frequency for one period instead of the standard period current, on every polarity inversion of the standard period current, and supplying a lamp current with the current value for the short period current higher than the standard period current. When a high pressure discharge lamp was lit up by the device, the arc spot is fixed and flickering on the screen could be suppressed.
[Patent Document 1] JP-A No. 2001-244088
That is, in a case of lighting a high pressure discharge lamp by AC, it is lit with opposing two electrodes being switched to a cathode and an anode alternately, and electrons projecting out of the cathode collide against the anode to generate discharge and the temperature of the anode hit by the electrons increases higher than that of the cathode.
Accordingly in a case of supplying the lamp current as described above, since an AC current is supplied under the identical condition between the electrodes and each of the electrodes is switched between the cathode and the anode in the same manner, temperature on each of the electrodes rises in the same manner, the temperature difference is not caused and, in addition, a high temperature portion is formed locally on the electrode to fix the arc spot.
However, high pressure discharge lamps or super-high pressure discharge lamps having a function of adding high value such as a light control function and an optional power fluctuation change have been manufactured recently, and it has been found that flickering can not be controlled effectively in such high pressure discharge lamps even when the lamp current as described above is supplied.
It is considered that a temperature difference is caused between the electrodes due to the structure of the electrode, the structure of the discharge lamp, effects caused by the reflection mirror, position for accessory equipments, and heat capacity or heat conduction property of them even when an AC current under identical condition is supplied between the electrodes.
As a result, the arc spot is not fixed tending to cause flickering, as well as a difference is caused to the amount of deformation due to deposition of metal vapors to the electrode surface or the abrasion of the electrode to result in the difference for the distance between the electrodes, thereby resulting in a problem that the control for the arc spot is made more difficult in a case of AC lighting.
In view of the above, the present inventor has made various experiments and, as a result, found that the electrode temperature can be kept constant by controlling the lamp current and, thus, the flickering can be moderated.
That is, in a case where the temperature difference is caused between the electrodes when a current is supplied under the same conditions between the electrodes, it has been found that a lamp current may be supplied to an electrode on a lower temperature such that the electrode functions as the anode for longer time and further that different optimal values are present for the ratio of time depending on the type of the high pressure discharge lamp respectively and the optimal values are equal so long as the type of high pressure discharge lamps is identical.
Further, it has been found that the flickering can be suppressed more effectively depending on the type of the high pressure discharge lamp by setting the ratio of time to an optimal value and using a ramp wave of gradually increasing or decreasing the current value before and/or after the polarity inversion of the short period current.